(1) Field of the Invention
The present invention relates to a flame-retarded polystyrene resin composition. More specifically, the present invention relates to a flame-retarded polystyrene resin composition having excellent heat resistance and good impact resistance and an article molded therefrom. Further, the present invention also relates to a flame-retarded polystyrene resin composition having a high heat distortion temperature under load and containing substantially no halogen and an article molded therefrom.
(2) Description of Related Art
A polystyrene resin is used in a wide variety of fields such as components of office automation equipment, home electric appliances and automobiles due to excellent impact resistance and moldability. However, its use is limited due to its flammability. As a method for making the polystyrene resin flame-retardant, addition of a halogen compound, a phosphorus compound or an inorganic compound as a flame retardant is known, so that a certain degree of flame retardancy is achieved by the addition. However, to increase the degree of safety of products, regulations on molded articles in office automation equipment and home electric appliances by a flame retardancy test based on SUBJECT 94 of Underwriters Laboratories (UL) Inc. which is a standard in U.S. have been becoming increasingly strict year by year, and a higher degree of flame-retardancy is desired.
Heretofore, as methods for improving flame retardancy of the polystyrene resin, a resin composition (Japanese Patent Laid-Open Publication No. 4-117442) comprising a polystyrene resin, a nitrogen compound such as melamine, a polyol and an organic phosphate and an ignition melt-dripping self-extinguishable polystyrene resin composition (Japanese Patent Publication No. 6-43542) comprising a rubber-modified polystyrene resin and a halogen compound are known, for example. However, these resin compositions have a problem that their applications are limited due to unsatisfactory flame retardancy.
In addition, in recent years, it has been reported that a halogen compound has an adverse effect on the environment, and a movement of use of halogen-free resin molded articles is thereby expanded from Europe. For this reason, demand for a halogen-free flame retardant has also been increased, and vigorous developments of halogen-free flame retardants for a variety of resins have been started. However, with respect to making the polystyrene resin flame-retardant without use of halogen, it has heretofore been believed to be difficult to achieve it due to flammability of the polystyrene resin.
As a known prior art in the field, a resin composition comprising a specific rubber-modified polystyrene resin and a phosphorus-containing flame retardant is disclosed in Japanese Patent Laid-Open Publication Nos. 8-176396 and 8-120152. To be more specific, it is disclosed that flame retardancy of melt-dripping self-extinguishing type is exhibited by use of triphenyl phosphate and derivative thereof or red phosphorus as the phosphorus-containing flame retardant. However, a composition using triphenyl phosphate and derivative thereof is caused to exhibit flame retardancy as a result of increasing flowability by a plasticizing effect so as to facilitate melt-dripping at the time of ignition. The resin composition has significantly low heat resistance and has a problem that it is of little practical use. In the case of a composition using red phosphorus, it needs to contain a relatively large amount of red phosphorus so as to exhibit flame retardancy. For this reason, a toxic phosphine gas is liable to be produced at the time of extrusion of the resin composition. Further, the resin composition also has such problems that red phosphorus is difficult to handle and that the obtained resin composition becomes brown inherent in red phosphorus, thereby limiting applications of the resin composition.
Further, Japanese Patent Laid-Open Publication No. 8-311278 discloses a melt-dripping self-extinguishing flame-retardant resin composition comprising a rubber-modified polystyrene resin, an organic phosphorus compound monomer, an organic phosphorus compound condensate and a silicone oil and containing 50 to 100% by weight of the above monomer in the organic phosphorus compound. More specifically, a rubber-modified polystyrene having a reduced viscosity of 0.53 dl/g is used, and the resin composition also has poor heat resistance and impact resistance and therefore has a problem that it is of little practical use.
Thus, in the conventional rubber-modified polystyrene resin compositions, a certain degree of flame retardancy is achieved, but they also have poor heat resistance. Particularly, it is difficult to apply them to applications requiring high heat resistance such as a housing of office automation equipment, and improvements in the resin compositions are desired.